Six starts into his D-backs career, Miller is 0-3 with an 8.49 ERA.
"This is frustrating because it's not even close to how good I can pitch," Miller said. "I feel like I'm letting my teammates down."
The theories run the gamut when it comes to Miller. It could be mental, it could be mechanical, or maybe he's just putting too much pressure on himself after the D-backs paid a large price to acquire him this past offseason from the Braves.
"Who knows?" manager Chip Hale said, when asked if he thought it was a mechanical issue. "I'm not a pitching guy. We're trying everything. We're working with him. Want him to stay positive. It's tough. It's tough. He's not very happy about it, and he's going to work at it, and we're going to get it right."
In his previous start against the Cardinals, Miller gave up five runs in five innings, but he at least made them do the damage with hits.
Sunday, with two runs already in, Miller walked pitcher Chad Bettis to load the bases and followed that with a free pass to Charlie Blackmon to force home a run.
"Those are the only two at-bats where I didn't feel like I was super comfortable," Miller said. "In those situations, I feel like I've been trying to place the ball almost. Trying to throw the perfect strike instead of just going after them and throwing hard. You see my [velocity] is 90 mph on those pitches, and it's not like I've been throwing in the past. I'm probably just letting up a little bit and babying those pitches. Just trying to throw strikes, which isn't what I do. I'm not a guy like [Zack] Greinke, who's going to throw the perfect strike at the bottom of the zone. That's not my game."
Instead, Miller is a pitcher with a four-seam fastball that can get hitters out up in the zone when he's letting it go rather than trying to spot it.
Miller said he feels like it could be a mechanical issue with him falling too far off the mound on his release, and he's worked between starts on standing taller during his delivery to combat that.
For now, it's back to the drawing board for Miller as he tries to sort it out before his next start, expected to come against his former club in Atlanta.
"No one's discussed it," Hale said, when asked if removing Miller from the rotation was an option. "That's not a subject that's been brought up. I don't think that's the answer, but it might be above me."
Steve Gilbert has covered the D-backs for MLB.com since 2001. Follow him
on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.